1. Field of the Invention
This invention relates to large mode field diameter optical fibers suitable for use in fiber lasers and amplifiers. More particularly, the invention relates to rare-earth doped optical fibers that have a large mode field diameter which is needed to insure low splice loss between component fibers and standard single mode fibers in doped fiber amplifiers (DFA) and fiber lasers.
2. Description of the Prior Art
Rare earths have been used as dopants of choice for the realization of fiber lasers and amplifiers since the first demonstration of gain in an Nd doped glass fiber, described by E. Snitzer, J. Appl. Phys. Vol.32, pp.36–39 (1961). In the telecommunications area, the invention of the erbium doped fiber amplifier (EDFA) revolutionized the design of optical communications systems. But erbium doped amplifying fiber characteristics have a major impact on the EDFA performance. Since the invention of the EDFA in the late 1980's, many erbium doped fiber designs have been developed to optimize EDFA performance. As there are different kinds of EDFA (pre-amplifier, booster amplifier, in-line amplifier, and the like) with different ranges of operating conditions, various fibers have been developed to accommodate specific needs.
However, there are common features inherent to the nature of the silica glass used as host of the majority of these fibers. All require a significant amount of aluminium to provide a broad emission spectrum and allow simultaneous amplification of a large number of wavelengths for DWDM applications. Aluminium also offers the advantage of reducing the tendency of the erbium atoms to cluster in the silica matrix, as clustering leads to the degradation of the amplifier performance. Moreover, aluminium increases the index of refraction of the core glass, thereby providing these fibers with a higher numerical aperture (0.18–0.3) than that of standard single mode fiber (SMF). High numerical aperture erbium doped fibers (EDF) can also be desirable because they have been shown to provide optimum gain efficiency in the EDFA. In order to preserve the single mode operation at both the pump (around 980 nm) and signal (around 1550 nm) wavelengths, EDFs have a small core size (typically 3–4 microns in diameter) and exhibit reduced mode field diameters (typically 4–5.5 microns at 1550 nm compared to 10 microns in a standard single mode fiber). This makes them difficult to splice and is a major source of loss, degrading the performance of the EDFA.
To increase the core diameter of a fiber laser, U.S. Pat. No. 5,937,134 proposes a composition of the core that, in addition to rare-earth has Ge, Al and P dopants and further has two cladding layers with the difference between the refractive index of the core and the first cladding layer being less than 0.008. This patent, however, appears to be restricted to cladding pumped fiber lasers and there is clearly a need for a large mode field diameter fiber that could be used not only in lasers but also in DFA and would allow the DFA designer and manufacturer to fabricate an amplifier with high efficiency and reduced noise level.